Commutation circuit for scr{3 s

ABSTRACT

An inverter circuit includes a pair of conventional commutating capacitors, coupled in series between the input conductors. Additional pair of commutating capacitors is provided, and an inductor is coupled between the common connection of the additional capacitors and the common connection of the conventional commutating capacitors.

United States Patent ,7 3 ,2 3,152 7/1966 walker George II. Studtmann Mount Prospect, 11]. 4,5315 3 Nov. 6, 1969 July 20,1971 Borg-Warner Corporation Chicago, Ill.

inventor Appl, No. Filed Patented Assignec COD/[MUTATION CIRCUIT FOR SCRS 10 Claims, 12 Drawing Figs. 0.8. CI... 321/45 R, 321/45 C Int. Cl "02m 7/48 Field of Search 321/43- References Cited UNITED STATES PATENTS 3,278,827 10/1966 Corey et a1. 321/44 3,311,809 3/1967 Corey et al 321/45 3,406,327 10/1968 Mapham et a1. 321/45 3,432,740 3/1969 Morgan 321/44 X 3,465,233 9/1969- Johnston et a1 321/45 3,487,278 12/1969 Tumbull et al.. 321/45 X Primary Examiner-William M. Shoop, .lr. Attorneys-Donald W. Banner, William S. McCurry and John W. Butcher ABSTRACT: An inverter circuit includes a pair of conventional commutating capacitors, coupled in series between the Input conductors. Additional, pair of commu tating capacitors connection of the additional capacitors and the common connection of the conventional commutating capacitors.

18 provided, and an inductor is coupled between the common PATENTEDJULZOIQYI 3,594,630

sum 2 or 3 I I I P J \nvenior 1,:36 eL George H. Srudtmonn Attorney PATENIED JUL20 I97! SIIKET 3 BF 3 Time Inventor George H. Studrmonn By AnJfiey appropriate polarity and-time duration to "allowit to regain its blocking stage; One

- normally coupled in-p'arallel with an closed and 'cla'imedfin U.S .1Pat: v titled Static Inverter System-With Energy- Return Circuit," which issued Man-7, l9 67;to"the assignee of this invention.

the commutating choke 7 However to date little progress has'been made in attemptsto means for receiving 1 not be derived from provided for applying the second DC voltage to the SCR as a comuurn'rioufcmcurr roitscir's BACKGROUND OF THE INVENTION In the inverter field semiconductor switches suchas silicon controlled rectifiers (S CRs) are frequently, connected for i the second DC voltage. Upon actuation .of the switching selective conduction, to'. provide an A Cout p'utvoltage from a a DC input circuit. To turn off or fcommutate an SCR, generally a reverse voltage is applied across the anode and cathode of .the previouslyconducting SCR. The reverse voltage is of an turnoff the SCR and way this turnoff is accomplished is to 'chargea commutating capacitor, which is v associated-SCR and one windingof thecommutating choke. when this SCR is gated on, the energy stored in" the commutatingcapacitor rapidly :discharges through the gated-onSCR andassociatedcommutating choke winding, inducingfavoltage by transformer action across the other commutating choke. winding to provide a turnoff. voltage for'the previously conducting, SCR.'-.Various techniques have been-utilized in anattempt to recover the, energy utilizedyto' minimum heating of the SCR's.

provideeffective commutation and One improvement in commutating'energ'y' recoveryis dis- No. 3,308,311 Studtmann, en-

The patent teachesthe recovery of. the commutating energy I either by a recovery circuit coupled to the commutating choke windings, or over-a spillover transformer'coupled in'. series with the loadconductor; or utilizing both circuits. A further and substantial" improvement in this art is described and claimed -in.U.S-.-Pat. No; 3,349,315 Studtmann, entitled Static Inverter System With Current Sharin'gfBy Both Commutating Choke windings J During." .Commutating Energy Recovery, which issuedjOct. 24; I967 to the assignee of this invention. Asthe. title implies, th'ec'ommutating'choke winding from-whichth'e load current is transferred at the beginning of commutation is puttowork or made to share the com mutating energy,'to minimize,the'current througheach of the commutating choke windings during the commutation intermeans in the commutation'circuit,3the first commutating capacitor-initially discharges as discharge of the second capacitorfisretarded by the inductor. Thereafter, the second cor'nmutating capacitor also discharges to augment the first capacitor discharge in providing the commutating pulse with minimum energy expenditure.

. THE DRAWINGS In the several figures of the d rawings like reference nu- 'merals identify like elements, and in the drawings: 1

FIG. l is a schematic representation, and FIGS. 2 and 3 are graphical illustrations, useful in understanding the general commutationproblem; e

' FIGS. 4 and 5 are schematic illustrations useful in un- -derstanding the widespread applicabilityof this invention;

FIG. 7' is a partial schematic diagram, and FIG. 8 is a graphiv FIG. 6 isa schematic diagram 'of a known inverter circuit;

cal illustration, useful in understanding. the general commutation operation;

' FIG. 9,-is a schematic diagram illustrating an improved Ccommutation circuit connected in accordance with this invention; FIG. l0'is a graphical illustration, to assist in understanding the improvement achieved by thepresent'invention;

" FIG. 11 is a schematic diagram of a preferred embodimen val. These-significant improvements have to do with recovery of the commutatingenergy after it has been "dumped" into to effect a commutation of the SCRs.

significantly reduce the-amount of energy utilized to. achieve effective commutation.

I It is 'thereforeaprimary consideration of this inventionto providea commutatinggcircuit for aninverter-which utilizes substantially less energy than previously expended andstill produces'effective commutation SUMMAR OF THE INVENTION An inverter circuit connected. in accordance jwith the present invention comprises a-pair of inputconductors for commutating circuitiscou'pled to the SCR, and includes I a second DC- voltage, which may or may the firstDC'voltage. Switching means is committing pulse, that is, in'the appropriate sense and for {the requisite time period to commutate the \SCRJParticuIarIy in accordance with this invention, circuit means is providedto shape the commutating pulse so as to minimize the amount of energy expended in commutatin'g the SCR.

" In a preferred embodiment of the invention such circuit means in the cornmutating circuit comprises a first capacitor one end coupled to ,one plate of the with aninducto'r having first capacitor..A second capacitor is also provided, having one plate'coupled to the other-end of the inductor and the other plate coupled tothe other plate of the first-capacitor. Initially capacitors are chargedtq approximately the level of of this invention; and V g FIG. l2'is aschematic diagram of another embodiment of the present invention. 1 I

' GENERAL INVERTER OPERATION Byway of general explanation,'an 12 is shown in FIG.-

. l coupledbetween an input conductor 13 and a normally .35

open switch". A batterysupplying a potential E is coupled between input conductors l3, 14; The symbol referenced l6 denotes acurrent generator: A current referenced I, flows through SCR [2 when the circuit is energized. A capacitor C is coupled between the movable contact of switch 15 and conductor 14. When the current flowing through the SCR is supplying a reactive load, such as a'motor, the reactive circuit tends to keep the current flowing even when the SCR is commutated. Hence in considering the inverter operation the current 1:, isconside'redto'be maintained, as represented by the current generatorl6, just before and just after SCR is commutated on.

i receiving a first DC voltage; A'power transfer circuit coupled 3 between the input conductors includes at least oneSCR. A I

' the dashed lines 17 and 18 in FIG. 2. I must be remembered that the actual cannot provide too high a din/d1. across AssumingJthat the capacitor C is charged to a voltage 2E,

that of the battery or inputvoltage, with the polarity indicated in'FlG. l, whenswitch 15 is closed this accumulated voltage is placed across the series circuit of the SCR and the supply battery so that the net voltage across SCR l2 is -E'. Considering the voltage supplied from the capacitor as that denoted e in FIG. 2, this voltage gradually decays as shown by the curve. At some time t the voltage on capacitor C has decayed below the battery voltage and at this time the SCR '12 is no longer back biased. It isrequisite that the voltage supplied by the commutating circuit, whethera capacitor or other arrangement, be greater than +E and of the appropriate sense time period I, which is The amount of energy U load 16 during the commutation interval reduced. The theoretical volt-time integral which can be used and still provide effective commutation is that'represented by However in practice it commutating voltage the SCR, and in additionthe commutating voltage must be at least slightly greater iturnofiT-time'; It is infthe reduction of the 'commutating energy nci suy directed; v

- FlGqfi depicts-jonerembodiment of theinventiombroadly V applied tocontntutate SCR l l'off with a minimurnenergy. ex-

r ni ea 'r o ns curve such as e'in FIG 2 to one more nearly proximatin'g the cu rve in FlG., 3, that the present invention .thanffrl-Eito provide a than reverent-a ivhichliniprovesthe penditure. .Asi therefshownwthe commutating circuit "2 I" is generally. represented. ivi'thin the 'broken'lines amincludes-a j pain -oti'cohductors-or circuitmeans' 22, 23 for receiving the I I spprtrehilcthatsthis second DCwolta'ge need not be. precisely.

-"biased ifor the'requilite turn off-time t Switch l 'i represents a switching'meansi for applying thesecond- DC voltage to the 1 Y ingfp ulse"includesiafirit capacitor 24, and an inductor 25 havin'g one ,end'coupledto one plate ofthe'first capacitor 24. A

g (the inductor 25-, and its other plate coupled over-"conductor 23 qio s v eb s flsim 24 I 1 f shown, capacitor 24, Zfiare charged 2 withsivitch l'sopen as J15; capacitor. initiallycdmmences to dischargein the itege p f a le'v e'l skilled in the art will.

" SC R. l-l t'or commutation. in the embodiment of Fl(i. 4, the circuit nieain'saior shapingiorproduciiig the desired commutatsecond-t:at)itcitrirz 6 has one platecoupled tothe other end of 'reason ofthe magnetic couplinghetween ivindings and of th'ejcommutatinglchoke 33, fromwinding '31 toyvinding 32.

Capacitor 36-,prcviouslyf charged vvith thev polarityindicated in FIG. 7 to the DC voltage E, then'discharges through the sei ries. circuit including winding 32and SCR 2 8 .'lt is apparent fronithe circuit that an additional discharge path forcapacitor 3 6-is provided as l,'flows' to theload. At' the same timecapacitor. 35- ischarging as' the charge current. flows from conductor l3 throughcapacitor 3 5, Some of the charge current. flows through-winding and SCRg28Uto-eonductorgld, and the,

remainder of the chargecurrent'flows overl load conductorf-30 to the load-.The combination of dischargeijandcharge currents producesaiturnofl' pulse across winding 32as' depicted by the".

curve 40, in FlG;- -'.'By transformeraetionof the commutating.

" choke the voltage H5: is raised to +25; producing'a'n' effective tumotfi pulse asIre'presented by curve I. The turnoff pulse voltage minus the battery or energizing voltage initially'provid'es.a"ttet-:-reverse-ljias voltage oreawna turnoff time I,

ends tvhen the turnoff ,pu'lse4l decays to a lev'el -l ETh'e en tire time interval 1, until the discharge pulse terminates is also "indicatedinl lfijr. 1

"it on be demonstrated thativitha hrid ge inverteras shown havingva centerrtapped commutating choke; the total energy I f reverse direction-ever switch- ,throu'ghSCRI Z; but capaeiw V f tor'lbatinotdischargeat-thesame rate'hecaus'eot' the retard-Q i i :ing' action ofi theindilctorl ln onesense it is appropriate ("although-notlit rally fiQt ect).to-describe the discharge ac v tion as capacitor 24 "first discharging tolmaintain the holdoff f clpltiitorfuis initially-higherjthan-that of capacitor s oot a p de h ssm n uonp 1 F IGQ 5 is gene 7 I I ,mut'atitig circuit ll-can be coupled directly across the SCR l2 rtst'eadzof across SCR, lliafnd thefenergizing-batt'ery E as i ted,to'coiiduct alterflatelyandenergize-theloadKiiot s \vn coupled to.,outputl'conductor-30 The SCR's arecouv seriqcircuitcomprises first SCRIZ', firstand second ,windings '31 "an 32" bf ecbrnmutating choke assembly 33,. an'd second main, to FIG. 4 but shoivs pledin'a series jeircuit Bet een input conductors 1s, 14. This connection"between-the firstiand 1 eecond cholte.svinriings3L3},isreferenced34; First and .value-ofthis-integrahor the estent'oifthe area under'curve 40. g g

" ae raiuabpasc airrlouos'rnaiuvaurlou energy. required iioLeffectiyeconimutation can be'sulastanto approximatelythe voltage, level 25. Upon closure of switch I U expended to holdofl the SCR fora specified time t, using an arbitrary voltage generator 2, t) -is given bythe' function r I I hffi-fiffi aflgffiQfil.., where e,,( i) represents the voltage" applied across one-half of the inducton foreitample acrosswindingfiz ill-1 1017; For

. y. successful commutation of the otherSCR'12.theyoltage'qu) voltages art .unll appiectatedhat the ,1 haigeofgboth'capacitors is simultaneous, but-the discharge.

.35. j mm st s v li-n f Qld t 1 tnimize the-amount of energy musteq'ual oreitceed the 'va' lue i-E-tor at least the time interval or FlG.-7 'theturnoff pulse is representedby curve in FlGQ v 8, and-the integralis representedl by a, summationjofall the area under thejjcurve-fl). Thus the; principal consideration of this invention is to'reduce the; turnoff energy [related to the t tially-reduced byfutilizing-the".netniork depictedin rogs. As

there showna secondeommutating capacitor 42 has one side coupled to. input conductor-:14, and the. other side coupled second'c'oinmut'ating capacitors 35 36. are coupled "in series I "bettlreei'l input condutjaors 13', 14;The common connection t t; thesefcomrnutatingcapacitorsis coupled to common conneetion' 34 between the cholte' windings 3131A pairot a energy return diodes 37,38, are coupled in series het'ween'the ajgiven momenta unidirectional potential t son .12, were winding 31,1 14 over conductor 30mm; o a. y

.. nsneqnauanvqis charged to substantially the inputDC vol;-

sea'v'gmh gm n cathodeoSCR28 to tlilftl ma as;- enema For a-reuonably'inductive losd (such-asamotor) ,the meetin s: I. remains substantially constant cluring the brief eomrfiiu'tationintervali This is-represented in FIG 7 and through a fiflF'intIuctorIS to the top ofcapacitor 36 and" to common.connection'lit with this 'arrat'tger'nent, the tum-on pulse' applied to -28 initially completes a path for capacit or discharge through 'yvinding 32 and SCR 28.. The transforrnjer action of commutating choke 33 provides avoltage of +25 the cathode of SCR l2, represented asthe uppermost value'in FIG. l0.1With"-a voltage of +Eapplied to the anode over conductor 13, SQR His-effectively hack-biased by a voltage .of Curve .44 t t-the lower partof FIG. 10

represents thedecay. of the back-bias voltage.

. .As SCR ZO ginsto conduct, there isinitially a rapid decay of the voltage on capacitor, represented by the firstpor'tion 45 of the upper curve s FIG-"l0. During this initial rapid discharge'ot capacitor Stithrough winding 32 and SCR 28, the energyin second capacitor '42 is momentarily held back or-re tardedby inductor 43.; Withappropriat'e design of this induc- .tor"'43', it .'-thereafter allows," capacitor 421m discharge I through inductor and windi 832 at a rate such that the ehergized'over'output conductor '30, it can 'beassu'rned that voltage. at c orrirno'n connectionflt'jis' maintained slightly greater than +8,- re p'resent ed bylthe portion of thecurve' in FIG; -l0,l t'or the timeinterval' Mhliinally both capacitors 42 and 36 dischargesubstantially completely as represented by I v r Those skilled inthea'rt \vill'appreciate he tumor; time oh at tli'efs'ar'ne time Ltheload 'urreiit l has'l been transferred, by 5 tainedtorthle SCR-s shownin.FlG.9, jaehieved1by'jthe delay: a

line components 36, 43 and 42, is substantially the same as that for the circuit depicted in FIG. 11. The capacitor size is split 'so that thevalues of capacitors 3S and 36 in FIG. 11 when added equal the value of the single capacitor 36 in FIG. 9. Such circuitidesign is now well understood in this art and requires no explanation at this point.

With the arrangement depicted in H6. 11, comparisons were made with conventional inverter arrangements such as that shown in FIG. 6, With both arrangements the value of E was 300 volts, and the load current l was 6.4 amperes. Each choke winding 31, 32 had an inductance value of 832 microhenries. in the arrangement of FIG. 6, each commutating capacitor 35, 36 had a value of 2 microfarads. With the circuit of FIG. 11, each commutating capacitor 35, 36 had a value of 0,416 microfarads, and each of the third and fourth commutating capacitors 42, 48 had a value of 0.918 microfarads. The first inductor 43 was a 66.5 microhenries choke. By evaluating the operation of the different circuits, it was found that reductions of 30 percent were obtained in the amount of energy required for efiective commutation, utilizing only one additional section of the delay line-type commutating circuit as shown in FIG, 11. Even more improvement is obtainable with additional sections. For example a reduction of 35 percent can be achievedwith the additional section as shown in FIG. 12, utilizing the fifth and sixth capacitors 50, 51 and the second inductor 52. However it is manifest that the principal energy reduction is achieved with the single section as explained in connection with-FIG. 9 and depicted in detail in FIG. 1 1.

While only particular embodiments of the invention have been shown and described, itwill be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the invention in its broader aspects. vFor example although the additional commutating capacitors are illustrated as connected to the same input conductors which energize the inverter, it is manifest that the commutating capacitors can also be coupled to separate energizing conductorsas is sometimes. done in-precharge" circuits. Therefore the aim, in the appended claims is to cover all such changes and modifications as may fall within the true spirit and scope of the invention.

What I claim is:

I. An inverter circuit comprising a pair of input conductors for receiving a unidirectional energizing potential,

a series circuit coupled between the input conductors comprising at least one SCR, a commutating choke assembly having first and second windings electrically and magnetically coupled to each other,'with a common connection between the first and second windings,

a first commutating capacitor coupled between one of the input conductors and said common connection, a second commutating capacitor coupled to said one input conductor, and a first inductor, coupled between said common connection and the second commutating capacitor, such that in the commutation of the SCR, the first commutating capacitor initially discharges through one of the commutating choke windings, and thereafter the second commutating capacitor discharges through the inductor and said one winding of the commutating choke.

2. An inverter circuit as claimedin claiml and further comprising a third commutating capacitor coupled to said one input conductor, and a second inductor, coupled between said third commutating capacitor and the common connection between the first inductor and=ithe second commutating capacitor, further to reduce the amount of energy required to commutate the SCR.

3. In an inverter circuit comprising a pair of input conductorsfor receiving a unidirectionalenergizing potential,

a series circuit coupled between the input conductors comprising a first SCR,a commutating choke assembly having first and second windings electrically and magnetically coupled to each other, with a common connection between the first and second windings, and a second SCR,

a first commutating capacitor coupled between one of the input conductors and said common connection, a second commutating capacitor coupled between said common connection and the other of the input conductors,

third and fourth commutating capacitors coupled in series between the input conductors, and a first inductor coupled between said common connection and a common connection between the third and fourth commutating capacitors, such that upon conduction of one SCR, one of the first and second commutating capacitors initially discharges through the associated commutating choke winding, and thereafter the corresponding one of the third and fourth commutating capacitors discharges through the first inductor and said associated commutating choke winding.

4. An inverter circuit as claimed in claim 3 and further comprising fifth and sixth commutating capacitors coupled in series between the input conductors, and a second inductor coupled between the common connection of the fifth and sixth commutating capacitors and the common connection between the third and fourth commutating capacitors, further to decrease the amount of energy required to commutate one of the SCRs.

5. An inverter circuit connected for energization over a pair of input conductors, comprising a series circuit coupled between the input conductors, in-

cluding a first silicon controlled rectifier, a second silicon controlled rectifier, and a commutating choke assembly having first and second windings electrically and magnetically coupled to each other,

a first commutating capacitor, connected to receive energy over a first energizing conductor and, upon gating on of the first silicon controlled rectifier, to discharge the stored energy into the first commutating choke winding,

; a second commutating capacitor, connected to receive energy over a second energizing conductor and, upon gating on of the second silicon controlled rectifier, to discharge the stored energy into the second commutating choke winding, and

third and fourth commutating capacitors coupled in series between the energizing conductors, and a first inductor coupled between the common connection of the third and fourth commutating capacitors and the first and second commutating capacitors, such that upon conduction of one silicon controlled rectifier, one of the first and second commutating capacitors initially discharges through the associated commutating choke winding, and thereafter the corresponding one of the third and fourth commutating capacitors discharges through the first inductor and said associated commutating choke winding.

6. An inverter circuit as claimed in claim 5 and further comprising fifth and sixth commutating capacitors coupled in series between the energizing conductors, and a second inductor coupled between the common connection of the fifth and sixth commutating capacitors and the common connection between the third and fourth commutating capacitors, further to decrease the amount of energy required to commutate one of the silicon controlled rectifiers.

7. An inverter circuit comprising a pair of input conductors for receiving a first DC voltage,

a power transfer circuit including at least one SCR coupled between the input conductors,'and

a commutating circuit, coupled to said SCR, comprising means for receiving a second DC voltage, switching means for applying said second DC voltage to the SCR as a commutating pulse in the appropriate sense to commutate the SCR, and circuit means for shaping the commutating pulse to minimize the amount of energy expended in the commutation.

8. An inverter circuit as-claimed in claim 7 in which the circuit means in the commutating circuit comprises a first capacitor, an inductor having one end coupled to one plate of the first capacitor and a second capacitor having one plate coupled to the other end of the inductor and the other plate coupled to the other plate of the first capacitor; which capacitors arefcharged to approximately the second DC voltage leyel prior'td actuation of the switching means such that upon ac-- tuation of the switching means, the first commutating capacitor initially discharges as the inductor retards discharge of the second capacitor, and thereafter the second commutating capacitordischarges. 9. An inverter circuit comprising apair of input conductors for receiving a unidirectional energizing potential, a series circuit coupled between the input conductors comprising at least one SCR, a commutating chokeassembly having first andsecond windingselectrically and magneti- .cally coupled to each other, with a common connection between the first and second windings, means including a precharge conductor for supplying a commutating potential,

' 1 a first commutating capacitor coupled to said common connection, a second commutating capaciton,and a first inductor, coupled between said common connection and the secondcommutating capacitor and further coupled to said precharge conductdr to complete a charging circuit for the first and second commutating capacitors, such that in the commutation of the SCR, th'efirst commutat ing capacitorinitially discharges through one of the comsot "isv 

1. An inverter circuit comprising a pair of input conductors for receiving a unidirectional energizing potential, a series circuit coupled between the input conductors comprising at least one SCR, a commutating choke assembly having first and second windings electrically and magnetically coupled to each other, with a common connection between the first and second windings, a first commutating capacitor coupled between one of the input conductors and said common connection, a second commutating capacitor coupled to said one input conductor, and a first inductor, coupled between said common connection and the second commutating capacitor, such that in the commutation of the SCR, the first commutating capacitor initially discharges through one of the commutating choke windings, and thereafter the second commutating capacitor discharges through the inductor and said one winding of the commutating choke.
 2. An inverter circuit as claimed in claim 1 and further comprising a third commutating capacitor coupled to said one input conductor, and a second inductor, coupled between said third commutating capacitor and the common connection between the first inductor and the second commutating capacitor, further to reduce the amount of energy required to commutate the SCR.
 3. In an inverter circuit comprising a pair of input conductors for receiving a unidirectional energizing potential, a series circuit coupled between the input conductors comprising a first SCR, a commutating choke assembly having first and second windings electrically and magnetically coupled to each other, with a common connection between the first and second windings, and a second SCR, a first commutating capacitor coupled between one of the input conductors and said common connection, a second commutating capacitor coupled between said common connection and the other of the input conductors, third and fourth commutating capacitors coupled in series between the input conductors, and a first inductor coupled between said common connection and a common connection between the third and fourth commutating capacitors, such that upon conduction of one SCR, one of the first and second commutating capacitors initially discharges through the associated commutating choke winding, and thereafter the corresponding one of the third and fourth commutating capacitors discharges through the first inductor and said associated commutating choke winding.
 4. An inverter circuit as claimed in claim 3 and further comprising fifth and sixth commutating capacitors coupled in series between the input conductors, and a second inductor coupled between the common connection of the fifth and sixth commutating capacitors and the common connection between the third and fourth commutating capacitors, further to decrease the amount of energy required to commutate one of the SCR''s.
 5. An inverter circuit connected for energization over a pair of input conductors, comprising a series circuit coupled between the input conductors, including a first silicon controlled rectifier, a second silicon controlled rectifier, and a commutating choke assembly having first and second windings electrically and magnetically coupled to each other, a first commutating capacitor, connected to receive energy over a first energizing conductor and, upon gating on of the first silicon controlled rectifier, to discharge the stored energy into the first commutating choke winding, a second commutating capacitor, connected to receive energy over a second energizing conductor and, upon gating on of the second silicon controlled rectifier, to discharge the stored energy into the second commutating choke winding, and third and fourth commutating capacitors coupled in series between the energizing conductors, and a first inductor coupled between the common connection of the third and fourth commutating capacitors and the first and second cOmmutating capacitors, such that upon conduction of one silicon controlled rectifier, one of the first and second commutating capacitors initially discharges through the associated commutating choke winding, and thereafter the corresponding one of the third and fourth commutating capacitors discharges through the first inductor and said associated commutating choke winding.
 6. An inverter circuit as claimed in claim 5 and further comprising fifth and sixth commutating capacitors coupled in series between the energizing conductors, and a second inductor coupled between the common connection of the fifth and sixth commutating capacitors and the common connection between the third and fourth commutating capacitors, further to decrease the amount of energy required to commutate one of the silicon controlled rectifiers.
 7. An inverter circuit comprising a pair of input conductors for receiving a first DC voltage, a power transfer circuit including at least one SCR coupled between the input conductors, and a commutating circuit, coupled to said SCR, comprising means for receiving a second DC voltage, switching means for applying said second DC voltage to the SCR as a commutating pulse in the appropriate sense to commutate the SCR, and circuit means for shaping the commutating pulse to minimize the amount of energy expended in the commutation.
 8. An inverter circuit as claimed in claim 7 in which the circuit means in the commutating circuit comprises a first capacitor, an inductor having one end coupled to one plate of the first capacitor and a second capacitor having one plate coupled to the other end of the inductor and the other plate coupled to the other plate of the first capacitor, which capacitors are charged to approximately the second DC voltage level prior to actuation of the switching means such that upon actuation of the switching means, the first commutating capacitor initially discharges as the inductor retards discharge of the second capacitor, and thereafter the second commutating capacitor discharges.
 9. An inverter circuit comprising a pair of input conductors for receiving a unidirectional energizing potential, a series circuit coupled between the input conductors comprising at least one SCR, a commutating choke assembly having first and second windings electrically and magnetically coupled to each other, with a common connection between the first and second windings, means including a precharge conductor for supplying a commutating potential, a first commutating capacitor coupled to said common connection, a second commutating capacitor, and a first inductor, coupled between said common connection and the second commutating capacitor and further coupled to said precharge conductor to complete a charging circuit for the first and second commutating capacitors, such that in the commutation of the SCR, the first commutating capacitor initially discharges through one of the commutating choke windings, and thereafter the second commutating capacitor discharges through the inductor and said one winding of the commutating choke.
 10. An inverter circuit as claimed in claim 9, further comprising a third commutating capacitor, and a second inductor, coupled between said third commutating capacitor and the common connection between the first inductor and the second commutating capacitor, further to reduce the amount of energy required to commutate the SCR. 